Julia Shaw hit traffic pay dirt earlier this week when she took to Slate to argue that twenty-somethings should follow her lead and get married now. Shaw got married at 23, and it seems to have worked out well for her. Amanda Marcotte responded by throwing some cold hard data on that argument, noting that women who marry later are less likely to get divorced and earn more, on average, than their earlier-marrying counterparts.

So should you wait to tie the knot? As tends to be the case in thorny areas like this, the evidence is decidedly mixed. In what should come as a shock to no one, the answer to when you should get married depends a lot on what you want out of a marriage, your career and life in general.

First, some throat-clearing. None of the data we have on marriage are definitively causal. That's a good thing. To have rock-solid evidence that marriage causes anything, we'd need to randomly require some people to marry at one age and others to marry at another age and then compare the results (and even that study design would have plenty of problems). Human Subjects Committees generally consider such studies unethical and don't let them happen.

So what we have are associations, or simple correlations between variables (say, age-at-marriage and income, or divorce, likelihood). That necessarily limits what you can say. "The kind of people who are going to marry younger are going to be somewhat different than those who marry older," says UVA sociologist Brad Wilcox, who runs the National Marriage Project and was a co-author of its "Knot Yet" report. People who marry younger tend to be poorer and less educated to start out with, Wilcox adds, as well as more religious.

So the finding that women who wait to get married make more money doesn't mean that a particular woman would earn more if she got married at 29 rather than 23. It could be that there's no effect of her age-at-marriage on income, and that the earnings differential between the groups just reflects other differences between their members.

As Marcotte says, the evidence is pretty persuasive that waiting to get married actually causes women's earnings to go up. For one thing, the difference holds up if you control for education level, as this chart from "Knot Yet" that Ezra posted indicates:

The asterisks and circumflexes indicate varying levels of statistical significance, but generally, the differences are statistically significant for high school graduates, those with some college, and college graduates. But they aren't for high school dropouts. There, you don't see any significant difference in earnings based on age at time of marriage.

What's more, the magnitudes involved are a bit smaller for high school graduates and those with only some college than for college graduates. That suggests that the benefits to waiting increase the more educated you are. And again, we don't have any evidence suggesting that waiting actually causes these differentials, and when they're as small as they are for the high school graduate and some college cohort, it could just be a quirk of demography. So it's not as simple as just "waiting makes you earn more." That seems to be true for college graduates, but the farther you go down the education ladder, the less clear the relationship looks.

The effects also decline the longer one waits. Getting married at 25 rather than 19 makes a big difference. At 30 rather than 25? Less so.

Earnings -- Men

But what we do know is that there is no such relationship for men:

No matter their education level, men who wait until they're 30 or older to marry earn a statistically smaller amount than men who marry earlier. This is interesting in light of research from the Urban Institute's Robert Lerman, among others, suggesting that men earn a "marriage premium." Lerman and his co-author, Avner Ahituv, found that marriage increases men's earnings by about 20 percent. But as Wilcox tells me, there's less evidence of a premium among women. Some studies find one, while others actually find a penalty, and there's a pretty consistent wage penalty for women who have children vs. those who don't.

That might partly explain the results you see in the above chart. If men make more money because they get married, then speeding up marriage could reap some economic dividends, enough to offset the disadvantages in terms of reduced flexibility when it comes to place and type of work.

So does waiting to get married increase your earnings? Probably, if you're a college-educated woman. For everyone else, it's less clear.

Happiness

Measuring happiness is a tricky business, and we've known for a while now that although life satisfaction constantly increases with income, its effect slows as one climbs the income ladder. Going from $100,000 to $120,000 a year creates a lot less happiness than going from $20,000 a year to $40,000 a year. Combine that with the murky economic data seen above, and you've got one messy picture.

"Knot Yet", the study Wilcox helped lead, has some interesting findings in this regard. He finds that self-reported happiness with one's marriage is highest for those who marry in their mid-20s, compared to those who do it in their late teens or early 20s or who wait until their late 20s or early 30s:

And it's not just feelings about the marriage. Among 24- to 29-year-olds, those who got married are less likely to get drunk frequently and to report that they're "highly depressed." Among 20- to 28-year-olds, married people are likelier to say they're "highly satisfied" with their lives:

Getting married earlier might also be better for your sex life, believe it or not. Dana Rotz at Mathematica Policy Research found that "a four year increase in age at marriage is associated with a couple having sex about one time less per month." In other words: the later you get married, the less sex you have.

Again, it could just be that happy people tend to get married earlier. When I noted the data showing that young married people are happier to Andrew Cherlin, a sociologist at Johns Hopkins and author of "The Marriage Go-Round," he replied: "I bet they are, because they’ve found good partners. That doesn’t mean that if you found people who are less well-matched to you, you should just marry them." The drinking and sex numbers in particular probably reflect the greater religiosity of early-married people as much as any happiness effects bestowed by marriage itself. Obviously you're going to be having more sex after getting married if you're religiously opposed to sex before marriage.

But the differences are still striking. It's uncontroversial at this point that marriage, in general, makes you happier, due to the work of Dartmouth's David Blanchflower. It seems that's true with twenty-somethings, too. That provides some evidence that you might want to get in on the fun sooner rather than later, if you have a good partner.

Conclusion

Marriage. It's what brings us together today. (20th Century Fox)

So it's complicated. As a wise man once said: "Everyone is different. No two people are not on fire." The best take-away to glean from this is probably that, all else being equal, being married makes you happier than you'd otherwise be, but it does so presumably because it involves spending a lot of your time with someone whom you love and who's a good match for you. "If you can find a marriage option that’s going to work and you’re in your early twenties, take it," Cherlin says. "The trouble is people are not finding those."

Holding partner quality equal, Wilcox argues, it still depends. "If your goal is to maximize your professional and financial accomplishment, then there’s no question that getting married later is the answer for you," he says. "But if you have a more traditional orientation in terms of having kids or being religious, then getting married and having kids in your 20s is a good bet."

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